U.S. Pat. No. 7,306,872 B2, issued Dec. 11, 2007, which is incorporated herein by reference, discloses a modular or repeating fuel cell cassette for forming a repeating unit in a solid oxide fuel cell stack (SOFC). The cassette comprises a housing having an electrode opening; a fuel cell subassembly including an electrode, the subassembly being mounted in the electrode opening such that either a cathode surface or an anode surface is exposed outside the cassette; inlet and outlet openings for providing fuel gas to, and removing spent fuel gas from, the anode surface, and for providing air to, and removing air from, the cathode surface.
The prior art SOFC repeating fuel cell unit is a cassette design comprising two thin sheet metal plates that are stamped and formed, then joined together with a metallurgical bond (laser weld) to form a hollow shell containing the thin fuel cell element. In its simplest form, this design has low mass and a low fabrication cost. However, in practice, the formed parts have very low rigidity and are prone to significant dimensional change by warping at the high operating temp (700° C. to 800° C.) of an SOFC. To mitigate this problem, several parts need to be added to the interior of the cassette to prevent dimensional collapse from compressive loads, which adds mass and cost. In addition, the forming process requires form radii, resulting in loose fitting components and fuel “bypass” channels that can have a significant negative effect on the fuel utilization of the cell. Thus, these bypass channels must be plugged with filler materials which also adds manufacturing complexity and cost. Further, the fuel cell subassembly itself is a load-bearing element of the load stack-up, and the so-called “picture frame”, to which the thin ceramic fuel cell is bonded, can induce shear stresses into the fragile cell which can lead to cell damage and reduced fuel cell efficiency.
What is needed in the art is an improved arrangement of plates for a fuel cell cassette wherein the fuel cell itself is isolated from thermal and compressive stresses induced in the metal components, and wherein the undesirable prior art bypass channels are eliminated.
It is a principal object of the present invention to improve reliability of a fuel cell stack.